Scintillation detectors are known from prior art. They comprise a scintillator, which can be solid or liquid, in which the radiation to be detected, in particular ionizing radiation, is at least partially absorbed. The absorbed radiation generates excited states in the scintillator which decay under emission of light.
The scintillation detector, moreover, comprises a light detector by means of which the light emitted by the scintillator is measured. In practice, the current light detector is a photo cathode in connection with a photomultiplier, whereby at first the light hits the photo cathode, from which it releases electrons which then are multiplied in the photomultiplier and are amplified such that an electric pulse which can be measured is generated. The present invention, however, can also be used for other light detectors.
Particularly, during use of scintillation detectors outside air conditioned rooms, as for example a lab, the temperature dependence of the amplification, in particular of the light detector, is negatively perceivable. The variation in amplification, in particular of the light detector, which is caused by the environment which is substantially, but not only, caused by different temperatures, therefore, has to be considered during the measurement.
It is known to calibrate the scintillation detector as a whole using a calibration source by exposing the detector to radiation of the calibration source, the characteristics of which, in particular its energy, are exactly known, and which adjust the measurement results to a reference value such that the detector is calibrated as a whole.
This calibration method has the disadvantage that for calibration real events have to be generated which actually overlay the values to be measured. This results in the radiation of low intensity which lies in the energy range of the calibration source, not being detectable or only rather poorly, because it is superposed by the calibration signals or it is completely overlapped. Is the scintillation detector, on the other hand, only calibrated prior and/or after the actual measurement, the response of the amplification due to environmental influences cannot be considered during the current measurement.
It is known that a light detector, in particular the combination of a photomultiplier with a photo cathode, can respond to environmental influences much faster and more significant than the scintillator itself such that a stabilization at least of the light detector during current operation is desirable. Therefore, in prior art the light detector is stabilized separately in that test signals which are generated by a test light source, for example an LED, are directed to the light detector. Regularly, this is carried out such that the light of the LED is coupled into the scintillator and hits the photo cathode of the photomultiplier over the scintillator.
The use of such test light sources enables stabilization of the light detector also during the current measurement of the signals to be detected, the so called effective signals. To not falsify the result therefore, however, it is necessary to separate the test light signals from the effective signals. For this, two methods are known.
To enable the separation of the test light signals from the effective signals in prior art it is suggested to use test light signals, which are larger than the largest and/or smaller than the smallest effective signals. By this, the test light signals measured by the light detector lie outside the spectrums of the effective signals, i.e., the effective spectrum such that these can be distinguished very easy from the effective signals.
This method is disadvantageous in that the dynamic range of the detector has to be larger than it would be required by the effective signals. This leads either to the possible detector resolution not being available or an increase in the electronic complexity for the detector, to increase the dynamic range for calibration purposes. Because the test light signal lies outside the range of effective signals, in principle, it is also not possible to verify the linearity of the characteristic curve of the detector. This is particularly disadvantageous with the use of photomultipliers with photo cathodes as light detectors, because these light detectors are not very linear, and the characteristic curve, moreover, changes with the supply voltage. Finally, in principle it is also not possible to consciously monitor a particularly interesting range of the amplitude or the pulse amplitude spectrum, because this range lies necessarily within the effective spectrum, and even in the middle of its particularly interesting range.
For avoiding these disadvantages it is known to use test light pulses generated by an LED, the amplitude of which is selected such that the pulses occur in the effective spectrum. Thereby, the test light pulses are pulsed in an adequate manner, whereby the test light generator additionally generates an electronic marking, a so called trigger signal, which is recognized and analyzed by the measurement circuit or the spectroscopic electronics of the detector, for example, in that an additional bit is set and read in an analog to digital converter (ADC). This marking enables then electronic separation between effective and test light signal.
This method is detrimental in that the test light generator and the measurement circuit of the detector have to be extraordinarily exactly tuned with respect to each other and, moreover, have to be coupled electronically. This leads to a substantially more complex electronic circuit as it would be necessary for the operation of the detector, associated with the requirement, to use more complex electronic components. From the use of additional circuitry, additional current and energy consumption results, this particularly is disadvantageous with mobile scintillation detectors, which are operated with batteries or accumulators.
Therefore, it is an object of the invention to avoid the disadvantages described above.